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The present invention relates to a method for producing Crxe2x80x94Nixe2x80x94Mo stainless steels having a high degree of resistance to localized corrosion. More particularly, stainless steels produced by the method of the present invention may demonstrate enhanced resistance to pitting, crevice corrosion, and stress corrosion cracking, making the steels suitable for a variety of uses such as, for example, in chloride ion-containing environments. These uses include, but are not limited to, condenser tubing, offshore platform equipment, heat exchangers, shell and tank construction for the pulp and paper industries, chemical process equipment, brewery equipment, feed-water heaters, flue gas desulfurization applications and use in the sea or coastal regions where the alloy may be exposed to marine atmospheric conditions.
Stainless steel alloys possess general corrosion resistance properties, making them useful for a variety of applications in corrosive environments. Examples of corrosion resistant stainless steel alloys are seen in U.S. Pat. No. 4,545,826 to McCunn and No. 4,911,886 to Pitler. Despite the general corrosion resistance of stainless steel alloys, chloride ion-containing environments, such as seawater and certain chemical processing environments, may be extremely aggressive in corroding these alloys. The corrosive attack most commonly appears as pitting and crevice corrosion, both of which may become severe forms of corrosion. Pitting is a process of forming localized, small cavities on a metallic surface by corrosion. These cavities are the result of localized corrosion and typically are confined to a point or small area. Crevice corrosion, which can be considered a severe form of pitting, is a localized corrosion of a metal surface at, or immediately adjacent to, an area that is shielded from full exposure to the environment by the surface of another material.
In testing and development of alloys of this kind, the corrosion resistance of an alloy may be predicted by its Critical Crevice Corrosion Temperature (xe2x80x9cCCCTxe2x80x9d). The CCCT of an alloy is the lowest temperature at which crevice corrosion occurs on samples of the alloy in a specific environment. The CCCT is typically determined in accordance with ASTM Standard G-48. The higher the CCCT, the greater the corrosion resistance of the alloy. Thus, for alloys exposed to harsher corrosive environments it is desirable for an alloy to possess as high a CCCT as possible.
Superaustenitic stainless steel alloys containing chromium and molybdenum provide improved resistance to pitting and crevice corrosion in comparison to prior art alloys. Chromium contributes to the oxidation and general corrosion resistance of the alloy. It also has the desired effects of raising the CCCT of an alloy and promoting the solubility of nitrogen, the significance of which is discussed below.
Nickel, a common element used in stainless steel alloys, is typically added for purposes of making the alloy austenitic, as well as contributing to the resistance of stress corrosion cracking (xe2x80x9cSCCxe2x80x9d). SCC is a corrosion mechanism in which the combination of a susceptible alloy, sustained tensile stress, and a particular environment leads to cracking of the metal. Typically, addition of nickel and molybdenum to a stainless steel increases its resistance to SCC as compared to standard austenitic stainless steels. However, the nickel and molybdenum-containing alloys are not totally immune from SCC.
Molybdenum may be added to a stainless steel alloy to increase the alloy""s resistance to pitting and crevice corrosion caused by chloride ions. Unfortunately, molybdenum may segregate during solidification, resulting in concentration of only two-thirds of the average molybdenum content of the alloy in dendrite cores. During metal casting, excess molybdenum is segregated into liquid metal ahead of the solidification front, resulting in formation of one or more eutectic phases within the alloy. In a continuous cast product, for example, this eutectic phase is frequently formed at or near the slab centerline. In many austenitic corrosion resistant alloys, the eutectic is composed of ferrite (body-centered cubic (BCC) Fexe2x80x94Cr solution) in addition to austenite (face-centered cubic (FCC) Fexe2x80x94Nixe2x80x94Cr solution) phases. For certain alloys compositions useful in connection with the present invention, the eutectic has been observed to be composed of austenite plus intermetallic phases. The intermetallic phase is typically sigma, chi, or Laves phase. Although sigma and chi phases have different structures, they may have similar compositions depending upon the conditions of intermetallic phase formation. These intermetallic phases, as well as other eutectic phases, may compromise the corrosion resistance of the alloy.
Nitrogen may typically be added to an alloy to suppress the development of sigma and chi phases, thereby contributing to the austenitic microstructure of the alloy and promoting higher CCCT values. However, nitrogen content must be kept low to avoid porosity in the alloy and problems during hot working. Nitrogen also contributes to increased strength of the alloy, as well as enhanced resistance to pitting and crevice corrosion.
Typically, the ability of an alloy to resist localized corrosive attack is critical in many industrial applications. Thus, there exists a need for a method of producing stainless steels that provide improved resistance to pitting and crevice corrosion. More particularly, there exists a need for a method of producing stainless steels that provide improved resistance to pitting and crevice corrosion at higher temperatures, as indicated by, for example, the CCCT.
The present invention addresses the above-described needs by providing a method for producing Crxe2x80x94Nixe2x80x94Mo stainless steels having improved corrosion resistance. In one form, the method includes providing an article of a stainless steel including chromium, nickel, and molybdenum and having a PREN greater than or equal to 50, and remelting at least a portion of the article to homogenize the portion. As examples, a portion, such as a surface region of the article, may be remelted, or the entire article may be remelted to homogenize the article or remelted portion. As used herein, PREN is calculated by the equation PREN=Cr+(3.3xc3x97Mo)+(30xc3x97N), where Cr represents the weight percentage of chromium in the alloy, Mo represents the weight percentage of molybdenum in the alloy, and N represents the weight percentage of nitrogen in the alloy. In one embodiment of the method, the Crxe2x80x94Nixe2x80x94Mo stainless steel comprises, by weight, 17 to 40% nickel, 14 to 22% chromium, 6 to 12% molybdenum, and 0.15 to 0.50% nitrogen.
The present invention further addresses the above-described needs by providing a method for producing such corrosion resistant stainless steels, wherein a melt of stainless steel including chromium, nickel, and molybdenum and having a PREN greater than or equal to 50 (calculated by the equation above) is cast to an ingot, slab, or other article, and is subsequently annealed for an extended period. The annealing treatment may be conducted prior or subsequent to hot working and is performed at a temperature and for a time sufficient to increase the homogeneity of (i.e. xe2x80x9chomogenizexe2x80x9d) the stainless steel. In one embodiment of the method, the stainless steel comprises, by weight, 17 to 40% nickel, 14 to 22% chromium, 6 to 12% molybdenum, and 0.15 to 0.50% nitrogen.
The inventors have determined that the method of the present invention significantly increases the Critical Crevice Corrosion Temperature (CCCT) of Crxe2x80x94Nixe2x80x94Mo stainless steels produced by the method without the increased costs of alloy additions. In addition, the method of the present invention enhances corrosion resistance without the effect on manufacturing operations associated with processing higher alloyed materials.
The present invention also is directed to corrosion resistant Crxe2x80x94Nixe2x80x94Mo stainless steels produced by the method of the present invention, and to articles formed of or including those steels. Such articles include, for example, plates and sheet.
The reader will appreciate the foregoing details and advantages of the present invention, as well as others, upon consideration of the following detailed description of embodiments of the invention. The reader also may comprehend additional details and advantages of the present invention upon making and/or using the method and/or the stainless steels of the present invention.